Dyeing by traditional water-based methods and subsequent washing processes produces large amounts of, usually strongly coloured, waste water. Furthermore, when dyeing, for instance, polyester fibres from an aqueous medium, the dyed fibres need to be subjected to a so called reduction clearing which causes additional effluent problems.
The aforementioned environmental drawbacks of water-based dyeing methods can be overcome by dyeing from supercritical carbon dioxide. Supercritical dyeing additionally offers the advantage that densities and viscosities in supercritical carbon dioxide are lower and diffusion more rapid than in liquids, shortening the process time.
The dyeing of substrate materials in liquid or supercritical carbon dioxide is well-known in the art. It is also known to employ reactive dyeing substances in supercritical dyeing methods that are capable of reacting with the substrate under the formation of a chemical bond. These reactive substances are usually derivatives of CO2-soluble disperse dyestuffs (chromophores) that contain a reactive group that is capable of reacting with specific residues in the substrate.
Unfortunately, supercritical dyeing methods employing the aforementioned reactive dyestuffs have been found to produce disappointing colour yields and to suffer from poor fixation of the dye to the substrate. Several attempts have been made to modify the dyeing methodology in order to obtain more satisfactory results with these reactive dyestuffs.
It has been proposed, for instance, to pre-treat the substrate prior to dyeing in order to enhance the reaction rate between the substrate and the reactive dye. One advocated approach is to chemically modify the substrate by treating the substrate with one or more reactants capable of reacting with reactive groups in the substrate. U.S. Pat. No. 5,578,088 describes a process for dyeing fibre materials comprising cellulose fibres or a mixture of cellulose fibres and polyester fibres, which comprises first modifying the fibre material with one or more compounds containing amino groups and then dyeing the modified fibre material with a fibre-reactive disperse dyestuff in supercritical CO2.
U.S. Pat. No. 5,298,032 describes a process for dyeing cellulose textile material with disperse dyes, which comprises pretreating the textile material at least 5% by weight of an auxiliary that promotes dye uptake and subsequently dyeing the pre-treated material with a disperse dye from supercritical CO2, the auxiliary being selected from the group consisting of a polyalkylene glycol, an alkanolamine and an aromatic compound with several hydroxyl groups.
Maeda et al. (Dyeing Cellulose Fibers with Reactive Disperse Dyes in Supercritical Carbon Dioxide, Textile Res. J. 72(3), 240-244 (2002)) describe the results of experiments in which cellulose fibres are dyed from supercritical carbon dioxide following pre-treatment with tetraethylene glycol dimethylether or N-methyl-2-pyrrolidinone, using reactive dyestuffs that comprise a triazine group for reaction with a hydroxyl group of cellulose fibres. The results show that pre-treatment improves the colour yield. The authors speculate that the pre-treatment solution can swell the cellulose fibres. In addition, the hypothesis that the pre-treatment solvents used are capable of forming hydrogen bonds with the cellulose chains which might help to prevent the complete deswelling of the fibres during the supercritical carbon dioxide treatment.
Japanese patent application 2002-201575 describes a method of dyeing a cellulosic fibre material, said method comprising pretreating the fibre material with a polar solvent capable of swelling the fibre and an alkali agent, followed by dying with a reaction disperse dye in a mixed fluid of supercritical carbon dioxide and a polar solvent such as ethanol, acetone etc. As examples of polar solvents capable of swelling the fibre ethylene glycol derivative and N-methylpyrrolidone are mentioned.
Despite the use of reactive dyestuffs and pre-treatment with reactants or organic solvents, known methods of supercritical dyeing have produced colour yields and wash-fastening properties that can be qualified as disappointing, especially in case these techniques are employed to dye cellulose fibres (e.g. cotton).